Today, photolithography requires short exposure wavelengths for successful imaging of very small semiconductor device dimensions. Photolithography systems must capture diffracted, off-axis light in order to resolve the fine features of an image since off-axis light contains spatial information needed to reconstruct the fine features, such as geometry line edges. Failure to capture the off-axis light within the system results in degradation of the quality of the image and loss of the ability to accurately reconstruct the fine features of the geometry being imaged.
FIG. 1 illustrates a graph of the diffraction angle of light passing through a slit, such as a transparent opening in a photomask, versus the relative intensity. When incident light encounters the slit, an illumination intensity profile, known as a Fraunhofer diffraction pattern, results that is a function of the slit width and the wavelength of the incident light. The central peak of the Fraunhofer pattern is typically known as the zero order peak and the order of each peak from the center increases by one, e.g., the first peak on either side of the central peak is a first order peak.
In a photolithography system, an imaging lens should capture the zero order peak and at least one higher order peak to create an accurate image on a wafer since the zero order peak contains the intensity of the image and the higher order peaks contain the image's spatial information. Reduced geometry sizes, however, require smaller transparent opening widths in the photomask. As the opening widths decrease, the angle of diffraction for the higher order peaks increases, which creates a need for an imaging lens that has a larger numerical aperture (NA). The relationship between the resolution of a photolithography system, e.g., the minimum feature size of an image, and the numerical aperture of the imaging lens is described by Rayleigh's formula, which may be expressed as:resolution=k1λ/NA where k1 represents the prefactor and λ is the wavelength of light emitted by a radiant energy source in the photolithography system.
As shown by the expression, resolution of a photolithography system is directly proportional to the wavelength and the prefactor and inversely proportional to the numerical aperture. The value of the prefactor, and thus the resolution of the associated photolithography system, is dependent upon the properties of the photoresist. As resists improve, the minimum feature size that can be satisfactorily imaged becomes smaller. Furthermore, as the wavelength of light is reduced by using a different light source, such as an Argon-Fluoride excimer laser instead of a Mercury arc lamp, the minimum feature size may be reduced even further. Higher resolution through increased numerical aperture requires optics that are physically larger. This, however, leads to practical design and manufacturing problems.
Other methods for increasing the capability of a photolithography system include using off-axis illumination. Referring to FIG. 2A, a schematic diagram of conventional normal incidence illumination system 10 is shown. Incident light 12 strikes mask 14 with an angle of incidence approximately equal to zero degrees with respect to normal. Light 12 passes through single slit 11 of mask 14 and is diffracted into zero order peak 16 and first order peaks 17 and 18. Zero order peak 16 has the greatest intensity and an angle of diffraction approximately equal to zero degrees. First order peaks 17 and 18 have lower intensities and respective angles of diffraction greater than zero degrees. Lens 20 may capture zero order peak 16 and project the image features present in zero order peak 16 onto a wafer (not expressly shown). Lens 20 may also project spatial information contained in first order peaks 17 and 18 onto the wafer if the numerical aperture of lens 20 is sufficiently large enough to capture light having angles of diffraction larger than zero degrees.
To satisfactorily image smaller device features, the width of slit 11 in mask 14 must be decreased. The smaller slit width causes the angle of diffraction of first order peaks 17 and 18 to increase. Therefore, capturing first order peaks 17 and 18 for smaller device features may require an imaging lens with a large numerical aperture. Large numerical aperture lens systems are currently being developed but are generally more costly to implement. As previously noted, physically larger lenses may lead to practical design and manufacturing problems.
Referring now to FIG. 2B, a schematic diagram of conventional off-axis illumination system 30 is shown. Incident light 32 strikes mask 34 with an angle of incidence greater than zero degrees with respect to normal. Incident light 32 passes through single slit 31 and is diffracted. Zero order peak 36 preferably has an angle of diffraction approximately equal to the angle of incidence. First order peaks 37 and 38 are diffracted at respective angles equidistant from zero order peak 36. Since incident light 32 has an angle of incidence greater than zero, first order peak 37 has an angle of diffraction less than zero order peak 36 while first order peak 38 has an angle of diffraction greater than zero order peak 36. The angle of incidence for illumination is chosen such that lens 40 may capture zero order peak 36 and first order peak 37, and project the image features present in zero order peak 36 and first order peak 37 onto a wafer (not expressly shown). Off-axis illumination system 30 may capture more spatial information than normal incidence illumination system 10. However, presently available off-axis illumination systems, such as off-axis illumination system 30, typically do not accurately reproduce all fine features of an image because first order peak 38 has a high angle of diffraction, which cannot be captured by lens 40 without using a much larger numerical aperture lens system.
In most conventional photolithography systems, a pellicle covers the photomask to protect the photomask from contamination. Conventional pellicles are typically designed to transmit on-axis light and attenuate off-axis light. On-axis transmission of light through a conventional pellicle may be maximized for one or more exposure wavelengths by manufacturing the pellicle to have an actual or physical thickness that is approximately equal to a design thickness that produces transmission maxima at the exposure wavelengths. As a result, conventional pellicles reduce the resolution of the associated photolithography systems.
Anti-reflective coatings applied on conventional pellicles have also been used to improve the on-axis and off-axis performance of the pellicle. The coating is typically tuned to reduce reflection from both surfaces of the pellicle and typically decreases the distance between the peaks and valleys of transmission versus wavelength. A conventional pellicle, however, is generally tuned to optimize the transmission of on-axis light, which results in the reduced transmission of off-axis light.